One problem with the concave "vessel" design of watercraft is that excess volume is required above the water line to accommodate unexpected cargo weight and to prevent catastrophic failure from water spilling into the vessel. That is, an open displacement vessel must have a large volume that increases from its bottom to top and presents a vertical or concave shape, which creates a maximum of friction at the water surface. Unfortunately, such a concave hull efficiently transmits and receives energy to and from the water surface. As a result, such watercraft lose much energy by wake formation and are easily rocked by waves.
Because of its hull shape, a vessel watercraft creates a wave (wake) simply by moving. This wave increasingly impedes boat movement as the boat acquires velocity, and becomes a standing wave at what is known as a limiting "hull-speed." This wake represents lost energy, which is flung away from the watercraft and hits beaches, other boat hulls and other structures. Besides greatly lowering watercraft speed, the wake forces the watercraft to adopt low speed within residential waterways and harbors to minimize its damage. Thus, the hull vessel design also wastes time and frustrates watercraft owners who desire quick transit of waterways and harbors, but are limited by the hull displacement problem of their boats.
The traditional vessel hull absorbs wave energy well. When a water wave meets a vertical or concave hull at air-water interface, some of the wave energy transfers to the hull. This energy slows boat movement, and can rock the boat when it is not moving, which can sicken the occupants. Consequently, a personal watercraft such as a pleasure boat less than 30 feet, and particularly personal trailorable watercraft (that is, less than 25 long) may become unusable if considerable wave activity is present. Waves only 2 or 3 feet high impart too much energy to such vessel hulls and even can fill the hull with water.
To overcome these vessel hull problems a boat is simply made larger. However, his solution is impractical for personal watercraft, especially trailorable watercraft that are limited in size by the dimensions of vehicles that can travel behind a car or truck on a highway. Furthermore, a watercraft, even in a body of water protected from the ocean, such as the Chesapeake Bay in the U.S. or Kobe Harbor in Japan can encounter such waves, and should be much larger to handle such waves.
One watercraft design that departs from the traditional vessel approach is a pontoon boat. A pontoon boat has a passenger compartment, or "platform" that sits upon two or three elongated air filled floats called "pontoons." A pontoon boat however, relies on a large flotation capacity of pontoons, which present a vertical surface at or near the water line. The vertical pontoon surfaces both absorb and generate waves and share generally the above-recited drawbacks with vessel designs. Furthermore, the pontoons create surface waves and typically are in a straight, non-optimal shape. Another disadvantageous feature of the pontoon boat design is that a motor is added between the pontoons (a separate source of drag) and is not integrated into the design itself. Yet another problem with a pontoon boat design is that most of the boat mass including the motor and power source is above water. A pontoon boat has a high center of gravity, which worsens the instability problem, particularly from waves, which collide perpendicularly to the elongated floats.
Houseboats are similar to pontoon boats and, unlike many open hull designs, have a high center of gravity above their center of buoyancy as described, for example, by Russell Conder, Handmade Houseboats, McGraw-Hill 1992 page 57. Because of its high center of gravity, a houseboat, like a pontoon boat generally is more susceptible to wave activity and is not usable in areas of high wave activity. Thus, a severe limitation of a pontoon design is that more of the weight is above the float than is desirable from the viewpoint of stability. Accordingly, it is important to keep the platform portion as close to the water surface as possible, in order to keep the center of gravity low.
Another design that deviates from the open hull approach is a semi-submerged catamaran or SWATH ship as, for example, described by Masuyama of Tokyo in U.S. Pat. No. 5,694,878, by Yoshida of Kobe in U.S. Pat. Nos. 4,763,596 and 4,986,204, and by Lang of California in U.S. Pat. Nos. 3,830,178; 3,897,744; and 4,944,238. The semi-submerged cargo ships described in these patents consist of submerged bodies having struts that support a hull, which can be convex. The submerged bodies tend to absorb less wave energy than does a concave or vertical displacement hull and also create less surface wave disturbances. However, these ships are not personal trailorable watercraft but have been exploited generally as huge ships designed for the open ocean. The inventors discovered that these ships have disadvantageous features that prevent their use as personal watercraft, as described below.
A semi-submerged catamaran as described, for example, by Masuyama has "no reserve buoyancy, so it needs a special lifting/diving system such as water ballasting equipment or lifting/diving plates in order to navigate or lie to with ample stability" as described in column 1 lines 41-44 of U.S. Pat. No. 4,763,596. More particularly, this and other patents in this area focus on the need to control the submerged parts of the boat because they do not have sufficient reserve buoyancy to automatically right the boat. Such complicated systems presented by these disclosures are not amenable to personal watercraft design, and certainly are not designed to prevent dipping of the boat in response to large shifts in cargo weight during boat movement.
The semi-submerged catamarans unfortunately also have a high center of gravity, particularly when loaded, and a large ratio of above-water volume to below-water volume. As explained above, these factors create instability, both to wave motion perpendicular to the boat and to perpendicular wind motion. A representative example of such a semi-submerged catamaran is shown in U.S. Pat. No. 4,174,671. As seen in figures of that patent, the majority of the craft volume, and particularly, the majority of the craft mass is above the water. Unfortunately, the semi-submerged catamaran has a high center of gravity that significantly exceeds the center of buoyancy and is not designed to carry a load that suddenly may shift position during transit. A personal watercraft on the other hand typically contains much reserve buoyancy and should have a center of gravity that is lower than the center of buoyancy to handle this problem. Thus, semi-submerged ships do not need a means to correct for catastrophic movements during travel, due to sudden and severe changes in loading.
Furthermore, a passenger on a large ocean going SWATH watercraft may represent small portion of the total mass and does not shift the loaded watercraft's center of gravity appreciably. A personal watercraft in contrast, is very sensitive to such changes in center of gravity. The inventors discovered that the traditional SWATH design is not amenable for use in personal (trailorable) sized watercraft of typically less than 30 feet long and particularly less than 25 feet long.
The drawbacks of the traditional vessel displacement hull design are worsened when combined with electric propulsion systems. Electric boats (and their cousins, fuel cell powered boats) require a heavy and bulky power source. The great weight of the power source requires yet a larger hull to displace a larger volume of water to keep the boat afloat. Thus, the development of electric watercraft technology in particular is hindered by the vessel hull problem. In fact, the vessel hull drag prevents economical use of electric motors for even moderate speed (above 8 mph) boating. This unfortunate state of affairs is a well known problem with electric boats, as, for example, stated in DOUGLAS LITTLE, ELECTRIC BOATS, THE QUIET HANDBOOK OF CLEAN, QUIET BOATING, (International Marine, 1994) on page 33 (referring to propeller design): "In the case of the electric boat, high speed-above 10 mph--is one factor that can be dropped immediately." Clearly, new designs are needed for wide acceptance of electric boats as general-purpose personal watercraft.
Efforts to overcome the hull problem for electric boats (and other boats) have emphasized streamlining the hull to make a larger middle section with tapered ends. However, hulls designed for semi-submerged catamarans, and their cousins, the submarine, generally contain straight conical midsections, which are not ideal, as explained by Guevel and Bardey, who describe an improved missile-like hull that gives constant pressure along its surface (U.S. Pat. No. 5,024,396). Other attempts to avoid inefficiencies from water contact with hulls are found in U.S. Pat. Nos. 5,544,610, 4,571,192 and 5,481,996. Heavy power supplies such as electric batteries and hydrogen fuel cell, when installed in an open hull watercraft, on the other hand, interfere with hull streamlining because of this greater need for both a bigger boat to hold the power supply and a reserve buoyancy for safety, as described above.
There is no comprehensive combination of these approaches that plays off the disadvantages of using heavy batteries or another low energy density power source with the other. In particular, no design strategy has successfully used the large mass and volume requirement of electric (or fuel cell) batteries as an asset, instead of a demerit, in a boat structure. Finally, there is no sufficient design for an electric powered and trailorable watercraft that can withstand moderate wave activity such as 2 feet high waves, without going to a larger vessel. A solution to these problems would open up new areas of boating to those that cannot buy or use the much greater size boats needed where such wave activity is present.